GENERAL CHEMISTRY OF MILK 81 



ethyl alcohol and amyl alcohol, but completely soluble in hot amyl 

 alcohol. 



Milk-fat is very susceptible to absorption of volatile sub- 

 stances. Undesirable flavors derived from fodder are, therefore, 

 readily imparted to milk and butter. However, volatile sub- 

 stances are promptly carried off by the intestinal tract of the ani- 

 mal and do not appear in the milk if milking is done several hours 

 after odoriferous food has been eaten. Therefore, if such food is 

 given immediately before or after milking the milk is not liable 

 to suffer. Even a strong flavor like that of onion will not be 

 noticeable unless it is derived from wild onions or leeks on the 

 pasture where cows graze for many hours during the day. 



Desirable flavors may be communicated to milk in the same 

 way. Clover hay, bran, and good pasture grass are known to 

 improve the flavor of milk and butter. 



The proportion of fatty acids in milk varies within hmits 

 according to the food taken, as has been pointed out. Some- 

 times fats are added to milk or butter with fraudulent intent. 

 The separation of the different acids for detection of fraud is a 

 difficult matter, and chemical tests for purity of butter-fat must, 

 therefore, depend on reactions of butter-fat as a whole. With 

 increasing quantities of low fatty acids the molecular weight and 

 the melting-point decrease, while the saponification number, the 

 Reichert-Meissl number, electric conductivity, and specific grav- 

 ity increase. With increasing quantities of unsaturated fatty 

 acids and cholesterin the iodin numbw and refraction number 

 increase, while the melting-point decreases with increasing amount 

 of oleic acid. The soluble and volatile acids are measured with 

 Polenske's number and the non-volatile acids with Hehner's 

 number. 



The Decomposition of Milk-fat 



Milk-fat is decomposed under the influence of light, oxygen, 

 and some kinds of micro-organisms, and then acquires a disagree- 

 able "rancid" taste and odor. The chemical processes which lead 

 to a rancid condition of fat are hydrolysis and oxidation. Hydro- 

 lysis produces fatty acids and glycerin, and the glycerin is then 

 further decomposed into water-soluble acids, aldehyds, and vola- 

 tile fatty acids. Unsaturated fatty acids are oxidized to hydroxy- 

 acids which are not volatile. 



Richmond gives the following reactions which result in the 

 rancidity of butter-fat. 



1. The glycerin is diminished on saponification. 



2. Soluble and volatile acids are increased. 



3. Free acids are greatly increased. 



